The present invention pertains to improved counter-rotating cylindrical members that are utilized on sewing machines for advancing a workpiece to the stitching instrumentalities of the machine so that a covering or safety overlock stitch may be formed along the line which joins or bonds together two faced portions of material forming said workpiece.
As is well known to those conversant in the art, machines which are adapted to form the type stitching mentioned above, the workpiece is caused to advance by the action of the counter-rotating cylindrical members and the space between said cylindrical members is such that a force is applied to the workpiece which assists in supporting it as it is being advanced.
When the sewing machines are used in a particular way, such for example, as forming a covering or safety stitch that is of substantial width or of the type which does not pass through the thickness of a spongy or extrememly elastic material that forms the facing material which is also of substantial thickness, the known forms of counter-rotating cylindrical members have not performed their intended function with complete satisfaction. This can be attributed to the fact that to apply a sufficient force to this type of workpiece, the counter-rotating cylindrical members must be located in positions of relatively close proximity which limits the thickness of the fold in the workpiece where the stitching is to be formed which consequently limits the width of the seam superimposed along the line which joins or bonds the pieces of material together that form the workpiece.
When the material forming the workpiece is that utilized in the fabrication of divers' suits, it is known that this material is of the closed cell type, i.e. it contains within its thickness substantially spherical shaped voids, which are mutually insulated and densely distributed in the mass of the material, and do not communicate with the exterior surface thereof. Usually this material is covered on one side with a synthetic fabric, which is generally nylon, so as to avoid direct contact with the human body. When fabricating these suits, it is common practice to unite the component parts together by means of bonding using a suitable adhesive or the like, as well as a reinforcing seam so as to maintain the garment in an impermeable condition.
The seam along the line of bonding, however, serves the purpose of providing reinforcement and preventing tearing apart and breakdown of the fabric structure.
In other words, the reinforcing seam should only cover the surface region of the bond.
The material for workpieces of the type described has a thickness which can vary from a minimum of 2.5 millimeters for certain lightweight diving suits up to more than 9 mm. for heavier suits. During incorporation of the seam, the workpiece is folded along the line of the bond with the folded portion being disposed between and advanced by the counter-rotating cylindrical members. In the case of material of substantial thickness, it is possible for the edge of the internal portion of the fold to be located relatively close to the base or lower edge of the cylindrical members and in combination with the deformation of the material there is a natural tendency for the workpiece to work itself out from between said cylindrical members. Additionally with the material of the workpiece having a substantial amount of elasticity and being subjected to the holding forces of the cylindrical members, the tendency of the workpiece to work free of said cylindrical members is greatly accentuated.
When forming such seams, the operator, in addition to guiding the material so that the line of bond will be centered in the stitching, must also support the material by holding it against the upper limiting guide which controls the depth of the stitching, i.e. the distance penetrated by the needle into the material from the external surface of the fabric.
The present known art consequently suffers from disadvantages which derive from the limitation of the internal folding of a workpiece which is necessary in order to avoid the tendency of said workpiece from dislodging itself from between the cylindrical members. The difficulties had during this operational procedure are due to the amount of force required for guiding this particular type of material, the limitations relative to the folded thickness of the material, as well as the width of seam obtainable and are very definitely related one to the other.
The present invention has corrected the disadvantages described above by providing a means which is capable of supporting the workpiece with a minimum of effort while it is being advanced between the counter-rotating cylindrical members which exert a minimum amount of force on the fold so as to form a desirable width of seam on the surface of the workpiece that is fabricated from a substantially elastic material.